'''

STACK

LIFO

All push and pop operations are to/from the top of the stack

PUSH an item onto the stack

POP an item off of the stack

PEEK to get item on top of stack, without removing it

CLEAR all items from stack

STACK USE CASE

Undo Command : track which command have been executed. Pop last command off the command stack to undo it

Calling Function: Function Calling in any programming language is managed using stack

Stack Class in Python:

list

collections.deque

queue.LifoQueue

BigO of stack:

Push/Pop element -----> O(1)

Search element by value -----> O(n)

'''

''' CLASS STACK() '''

class Stack():

def __init__(self):

self.stack = list()

def push(self, item):

self.stack.append(item)

def pop(self):

if len(self.stack) > 0:

return self.stack.pop()

def peek(self):

if len(self.stack) > 0:

return self.stack[len(self.stack)-1]

else:

return None

def __str__(self):

return str(self.stack)

''' CLASS STACK() END '''

stack = list()

# .append method to append items to the stack

stack.append(1)

stack.append(4)

stack.append(9)

stack.append(16)

print(f"Stack is {stack}", end="\n\n")

stack.pop()

print(f"Deleted Last item of Stack (LIFO) {stack}", end="\n\n")

# Calling Stack() class

stack = Stack()

stack.push(1)

stack.push(2)

stack.push(3)

stack.push('B')

print(f"New Stack is {stack}")

print(f"Deleted item is {stack.pop()}")

print(f"Seek is at {stack.peek()}")

#TODO: Stack continue ( https://youtu.be/kQDxmjfkIKY?t=3077 )